The evolution of light imaging devices and light detectors of the vacuum tube variety has led to a simple cylindrical tube of diameter much larger than its thickness known as a wafer tube. This design provides the largest area for imaging and the smallest volume for ease of evacuation, as well as economy of space in a system. To reduce contact resistance and increase contact reliability two or more toroidal electrodes of slightly different diameters are mounted on the outside of the narrow circumferential wall and sealed therethrough to provide inner terminals, leaving both circular tube walls free for optical purposes, if needed. Electro-optical targets such as photocathodes, photodiodes, photoconductors, electron multipliers and phosphor screens are mounted in the tube and connect to these inner terminals.
To test such devices it is usual to set up a standard source and a socket with spring contacts a fixed distance therefrom, into which contacts the tubes are inserted and tested. Such a setup is not suitable in critical tests of experimental devices for a number of reasons. First the resistance of the spring contacts can vary causing the applied voltage or signal extracted (depending on the device under test) to vary from test to test. The distance from the source to tube can also vary. It may be desirable to have the light incident from opposite directions for particular measurements. And finally the augular orientation of tube can also vary in tests where comparison of elemental areas of the target surface are of interest.